This invention relates to a optical sensor for detecting fault points and fault sections, for example, at electric power transmission lines, distribution lines and transformer substations and the like, and a method for producing the optical sensor.
Optical sensors have been known, for example, as disclosed in Japanese Patent Application Laid-open No. 62-54,170. As shown in FIG. 1, the disclosed known optical voltage sensor includes an optical fiber core 1, a plug 2, a microlens 3, a polarizer 4, a Pockels element 5, a quarter-wave plate 6, an analyzer 7, a reflecting mirror 8, a microlens 9, a plug 10 and an optical fiber core 11. A light source (not shown) generates light which passes through optical elements 1-11 of the sensor, which light is received by a light receiver (not shown). The voltage applied to the Pockels element 5 is optically measured from the modulation of the light received by the light receiver.
In the optical voltage sensor shown in FIG. 1, the respective optical elements 1-11 are accommodated in a package 12 having therein a holder 13. The holder 13 is provided with a pipe 14 for holding the plug 2 and the microlens 3 on the input side and with a pipe 15 for holding the microlens 9 and the plug 10 on the output side.
In order to prevent the moisture from entering the optical passages and to stabilize the detector against vibrations due to drop shock, the microlens 3, the polarizer 4, the Pockels element 5, the quarter-wave plate 6, the analyzer 7, the reflecting mirror 8, and the microlens 9 are joined to one another by means of a flexible adhesive 16a at boundaries on both sides of the Pockels element 5 and by means of a rigid adhesive 16b at remaining boundaries as shown in FIG. 2. Moreover, in order to unite these elements and the package 12, and more improve the resistance to the moisture and the vibrations due to drop shock, the respective optical elements are coated therearound with a flexible resin 17 and embedded in a rigid resin 18 by potting. Moreover, the package 12 is filled with Kevlar fibers 19 on the side of the optical cores 1 and 11 with respect to the holder 13.
In the conventional optical voltage sensor described above, however, since the respective optical elements are bonded with each other by means of the adhesives 16a and 16b, residual stresses occur in the boundaries between them due to shrinkage of the adhesives when solidifying. Therefore, the optical elements may often be shifted from each other owing to change in external temperature, thereby increasing light loss and causing changes in modulation. As a result, temperature characteristics of the instrument are detrimentally affected.
Moreover, since the respective optical elements are coated with the flexible resin 17 and further embedded in the rigid resin 18 by potting, cracks may occur in the boundaries between the optical elements owing to thermal stresses caused by temperature changes due to difference between coefficients of thermal expansion of the flexible and rigid resins 17 and 18. Further, cracks may occur in the flexible and rigid resins 17 and 18. As a result, the optical elements may become susceptible to moisture and vibration due to drop shock.
With the optical voltage sensor described above, moreover, all the optical elements are accommodated in the package 12 having the holder 13 which holds the plug 2 and the microlens 3 by the pipe 14 on the input side and the microlens 9 and the plug 10 by the pipe 15 on the output side. Therefore, the number of parts is unavoidably high, and the holder 13 and the package 12 fitted together must be produced with high accuracy. Accordingly, the instrument itself is expensive and the process for producing the instrument is complicated, thus lowering mass-productivity.